A color filter used in a liquid crystal display, an imaging device or the like element generally has a construction in which pixels in a mosaic or stripe form are arranged on a transparent substrate such as a glass and has been hitherto prepared by a dyeing method (T. Uchida et al, Record of International Display Res. Conf. (1982), 166, IEEE Trans. ED-30 (1983) 503), an electrodeposition method (M. Suginoya et al, Proc. Japan Display '83 (1983 206) or a printing method (Tsuda et al, Proceedings of 13th Liquid Crystal Forum (1987) 22).
According to the dyeing method, a perchromate or a water soluble azide is added to a water soluble polymer such as gelatin, casein or polyacrylamide to impart photosensitivity thereto. After exposure, a development step is performed to obtain a relief pattern which in turn is colored to obtain a colored image. By repeating the coloring step for red, blue and green colors to obtain a color filter. To prevent color mixing, a coloration preventing treatment with tannic acid is performed between the respective coloring steps or an intermediate layer of an acrylic resin is formed. Thus, the dyeing method includes complicated steps, so that it is difficult to control the color tone. Further, the dyeing method has drawbacks that the water soluble polymer which is a dyeing medium has poor heat resistance and the color has poor heat resistance and poor weatherability.
To solve the above defects, studies have been made on the electrodeposition method, printing method and pigment dispersing method. With the electrodeposition method, however, the colored film is so thick that there are caused problems such as a large voltage drop and a limitation in pattern arrangement. The printing method has drawbacks because of poor dimensional accuracy and poor surface smoothness.
To solve the drawbacks of the conventional methods, there is proposed a pigment dispersing method. In this method, a pigment is dispersed in a photosensitive resin and the dispersion is applied onto a substrate. After drying, exposure and development are carried out to obtain a colored image. Thus, this method has a merit that it enables to obtain color filters by a simpler process in comparison with the dyeing method. A variety of photosensitive resins have been proposed for use in the pigment dispersing method, such as photosensitive polyimide resins disclosed in JP-A-60-237403, photosensitive resins composed of an acrylic polymer and an azide compound and disclosed in JP-A-1-200353, 4-7373 and 4-91173, photosensitive resins composed of an acrylate monomer, an organic polymer binder and a photopolymerization initiator and disclosed in JP-A-1-152449, and a chemically amplified-type photosensitive resins composed of a phenolic resin, a crosslinking agent having an N-methylol structure and an agent capable of generating an acid upon irradiation with a light and disclosed in JP-A-4-163552.
While a high heat resistance is obtainable when a photosensitive polyimide is used as the photosensitive resin for the pigment dispersing method, there are drawbacks because of low sensitivity and of necessity to use an organic solvent in the development step. With a conventional system in which an azide compound is used as the photosensitive agent, there involve problems because of low sensitivity, poor heat resistance and susceptibility to oxygen during the exposing step. A system in which radical polymerization of an acrylate monomer is utilized poses a problem that the exposure step is affected by oxygen, so that the sensitivity is reduced considerably, though the sensitivity is high. To avoid the influence of oxygen, therefore, it is necessary to form an oxygen barrier film or to perform the exposure in an inert atmosphere. But this requires a complicated process and an expensive apparatus. The chemically amplified-type photosensitive resin for forming a image by a catalytic reaction by an acid generated upon exposure can give high sensitivity and is not adversely affected by oxygen during exposure. However, there involves a problem because it is necessary to introduce a heating step between the exposure and development steps and because the sensitivity and the pattern shape are affected by a period of time from the exposure to the development, so that it is difficult to control the process. Further, the conventional photosensitive resin composition has drawbacks that the stability during storage is not good.
On the other hand, the conventional pigment-free photosensitive resin composition has the same drawbacks as those in the above-mentioned, photosensitive resin composition used for the preparation of a color filter by using the pigment dispersion method. In the conventional pigment-free photosensitive resin composition, sufficient consideration has not been taken for the chemical resistance, heat resistance and transparency of a cured body obtained through an exposure step. Thus, when the conventional pigment-free photosensitive resin composition is used as a protective material for a coloring material layer, there are caused problems of poor heat resistance and poor chemical resistance, so that a considerable change of color occurs by heating and peeling occurs upon contact with chemicals. Under the existing circumstances, further improvement is desired.
It is, therefore, an object of the present invention to provide a photosensitive resin composition which has such excellent characteristics that it has high sensitivity, it does not require a heat treatment between exposure-development steps, it causes little change of sensitivity and pattern shape with time from the exposure to the development, it has high heat resistance, it permits the use of an aqueous system in the development step and it has good storage stability.
It is another object of the present invention to provide a pigment-free, photosensitive resin composition which has good heat resistance and good chemical resistance and which has a long shelf life.